The United States Geologic Survey (USGS) reported in 2008 that undiscovered
technically recoverable oil in the Bakken was about 3.6 billion barrels across
the U.S. portion of the basin, considering recent successful application of
horizontal wells and multistage hydraulic fracturing technologies. As the
development of the unconventional resources in the Williston Basin continues
beyond the phases of exploration and lease evaluation, optimum well spacing and
recovery factor will become forefront considerations in the formulation of
asset development strategies. Based on our studies the reservoir producing
mechanism is primarily solution gas drive and primary oil recovery factor is
lower than 15% of the original oil in-place. This low recovery or very high oil
volume remaining in place is a strong motivation to investigate the application
of enhanced oil recovery methods in this basin.
This paper describes the construction of numerical simulation models using
typical fluid and rock properties for the Bakken and Three Forks, assuming both
naturally fractured and single porosity systems and their combinations.
Multistage hydraulic fracture properties are determined from well completion
engineering and coupled with the flow models. The flow models are constrained
by well operating practices implemented by operators across the basin during
primary oil production.
The results of pressure maintenance methods to arrest the rapid reservoir
pressure decline due to large pressure drawdown necessary to produce oil and
water, as well as gas (including carbon dioxide) and water injection methods to
improve oil recovery are presented.
The Bakken Formation in the Williston Basin has three members, namely the Upper
Bakken Shale, Middle Bakken and Lower Bakken Shale. The Lodgepole overlays the
Upper Bakken Shale while the Lower Bakken Shale overlays the Three Forks
Formation, Figure 1. Both the Upper and Lower Bakken Shale are organic rich
dark shale which are both the source and traps for the hydrocarbons. The Three
Forks formation also has three members; the Upper, Middle and Lower Three
Forks. All three members are highly laminated dolomite/claystone/shale
sequences. The Upper Three Forks has the best reservoir quality of the three as
water cut for wells completed in these zones generally increases with depth.
The thickness of the Bakken and the Three Forks increases toward the center of
the Basin along the Nesson Anticline. The Middle Bakken and Upper Three Forks
with the best reservoir quality (porosity and oil saturation) are the primary
targets for development.